WO2002041507A2

WO2002041507A2 - Two-level u-shaped groove and method of fabricating the same

Info

Publication number: WO2002041507A2
Application number: PCT/US2001/043881
Authority: WO
Inventors: Glenn E Kohnke; Lu Chen; Yuri Suzuki
Original assignee: Corning Incorporated; Cornell Research Foundation Inc.
Priority date: 2000-11-20
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: AU2002239320A1; WO2002041507A3

Abstract

A structure is provided for coupling optical devices in an integrated optical microcircuit. The structure comprises a substrate (10) having a groove formed in one surface (12) thereof. The groove has a bottom substantially parallel to the surface. The grove also has an upper side wall adjacent the surface, the upper side wall being substantially perpendicular to the surface and defining an upper groove width (Wu). The groove also has a lower side wall adjacent the bottom, the lower side wall being substantially perpendicular to the surface and defiing a lower groove width (Wl). The upper groove width (Wu) is greater than the lower groove width (Wl). A process for fabricating the structure is also provided which uses Bosch etching to form the vertical sidewalls of the two-level U-shaped groove.

Description

TWO-LEVEL U-SHAPED GROOVE AND METHOD OF FABRICATING THE SAME

Field of Invention

The present invention relates to a two-level U-shaped groove for aligning optical waveguides, such as optical fibers, and a method of fabricating the same.

Background of the Invention

Integrated optical microcircuits require the accurate alignment of the waveguides of the optical system, which most frequently take the form of optical fibers. One method of achieving optical alignment is through the use of V-shaped grooves formed in the substrate of the optical circuit. The optical waveguides are positioned in the V-shaped groove, or adjacent thereto, to provide alignment. Fig. 1 illustrates a V-shaped groove 1 extending from a surface 12 of substrate 10.

V-shaped grooves are commonly formed in the substrate surface by etching a silicon substrate using a wet potassium hydroxide (KOH) etch. KOH wet etches are anisotropic, in which the planes of bulk silicon act as etch stops, the slowest etching rate occurring along the <111> plane, thereby forming a V-shaped cross section. Unfortunately, the use of KOH etching provides several disadvantages. It is difficult to control the etching rate of the substrate. In addition, KOH etching can cause an undercut of the pattern. As a result, KOH etching does not allow for precise control of the etching dimensions and, thus, is generally limited to uses having a low- resolution requirement. In addition, as a result of the anisotropic nature of KOH etching, slight misalignment of the silicon wafer with respect to the crystal lattice directions of the substrate can cause the misalignment of the etching patterns. More specifically, in the pattern design, the grooves formed by a KOH wet etch must be designed along the <111> direction on a <100> silicon wafer. During pattern alignment, a small rotation of the wafer will result in misalignment of two adjacent V-shaped grooves, as illustrated in Fig. 2. Thus, KOH etching requires stringent pattern alignment to achieve alignment between two adjacent V-shaped grooves.

Furthermore, KOH etching requires the use of a Si N₄ layer as the hard mask to protect areas where etching is not desired. Unfortunately, use of a SbN4 layer is incompatible with some processing steps.

The shortcomings of KOH etching create obstacles to forming V-shaped grooves with finer resolution structures, especially optical elements formed using electron beam (e-beam) lithography or x-ray lithography that push feature limits.

Another approach is the formation of single, rectangular, U-shaped grooves in a substrate. Figs. 3 and 4 illustrate single U-shaped grooves 1' extending from a surface 12 of substrate 10 in cross sectional and angled top down view, respectively.

Single U-shaped grooves can be formed using dry etching techniques. Unfortunately, the single U-shaped groove is not tolerant to slight variation of optical fiber diameter. More specifically, despite the high anisotropy of the dry etch that is used in the fabrication of these rectangular grooves, the edges of the groove have an uncontrolled slope which makes it difficult to create a tight fit for a particular optical fiber. While anisotropic plasma etching techniques provide accurate control of lateral dimensions, these techniques do not afford good depth control beyond about 0.5 micron. Therefore fiber placement is only accurate to within 0.5μm in the vertical direction.

There remains a need, therefore, for a structure that provides precise control for positioning and aligning optical fibers. There also remains a need for a method of forming such a structure that is independent of the silicon substrate orientation and that is compatible with other processes.

Summary of the Invention

The present invention provides a structure for optically coupling optical devices. The structure comprises a substrate having a groove formed in one surface thereof. The groove has a bottom substantially parallel to the surface, an upper side wall adjacent the surface, the upper side wall being substantially perpendicular to the surface and defining an upper groove width (wu), and a lower side wall adjacent the bottom. The lower side wall is substantially perpendicular to the surface and defines a lower groove width (wi), the upper groove width (w_u) being greater than the lower groove width (wi).

The present invention also provides a method for fabricating a structure for optically coupling optical devices. The method comprises the steps of:

(a) etching a silicon substrate to form a U-shaped groove extending from one surface of the substrate a depth (dι), the groove having a bottom substantially perpendicular to the surface and a lower side wall adjacent the bottom, the lower side wall being substantially perpendicular to the surface and defining a lower groove width (wi); and

(b) further etching the substrate at said U-shaped groove a depth (du) to form an upper side wall adjacent the surface, said upper side wall being substantially perpendicular to the surface and defining an upper groove width (w_u) superimposed over said lower groove width (wi), wherein the upper groove width ( u) is greater than the lower groove width (wi).

A preferred method for fabricating a two-level U-shaped groove structure for optically coupling optical devices comprises the steps of:

(a) forming a silicon oxide layer on a surface of a silicon substrate; (b) forming a resist on the silicon oxide layer such that a portion of the silicon oxide layer is exposed to define a lower groove width (wi);

(c) removing the exposed portion of the silicon oxide layer to expose a portion of the silicon substrate; (d) removing the resist;

(e) forming a second resist on the silicon oxide layer such that a portion of the silicon oxide layer adjacent the exposed silicon substrate is exposed to define an upper groove width (Wu) superimposed over the lower groove width (wi);

(f) etching the exposed silicon substrate; (g) removing the exposed portion of the silicon oxide layer; and

(h) etching the exposed silicon substrate.

Brief Description of the Drawings

The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures.

Fig. 1 illustrates, in a cross sectional view, a V-shaped groove formed in a silicon substrate.

Fig. 2 illustrates, in an angled top down view, two V-shaped grooves formed in a silicon substrate.

Fig. 3 illustrates, in a cross sectional view, a single U-shaped groove formed in a silicon substrate.

Fig. 4 illustrates, in an angled top down view, two U-shaped grooves formed in a silicon substrate. Fig. 5 illustrates, in a cross sectional view, a two-level U-shaped groove formed in a substrate according to the present invention.

Fig. 6 illustrates the structure of Fig. 5 having an optical fiber positioned in the two-level U-shaped groove,

Fig. 7 is a scanning electron micrograph (SEM) illustrating, in an angled top down view, two adjacent two-level U-shaped grooves.

Fig. 8 is a scanning electron micrograph (SEM) illustrating, in an angled cross sectional view, two adjacent two-level U-shaped grooves having an optical element positioned between the grooves.

Fig. 9 illustrates, in a cross sectional view, a structure having a substrate, a silicon oxide layer disposed on the substrate, and a resist disposed on the silicon oxide layer such that a portion of the silicon oxide layer remains exposed and defines a lower groove width (wi).

Fig. 10 illustrates the structure of Fig. 9 wherein the exposed silicon oxide layer has been removed.

Fig. 11 illustrates the structure of Fig. 10 wherein the resist has been removed and a second resist is disposed on the silicon oxide layer such that a portion of the silicon oxide layer adjacent the exposed silicon substrate remains exposed and defines an upper groove width (w_u).

Fig. 12 illustrates the structure of Fig. 11 wherein the exposed silicon substrate has been etched a depth (di).

Fig. 13 illustrates the structure of Fig. 12 wherein the exposed silicon oxide layer has been removed.

Fig. 14 illustrates the structure of Fig. 13 wherein the resist has been removed and the exposed silicon oxide layer has been etched a depth (du). Fig. 15 illustrates the structure of Fig. 14 wherein the silicon oxide layer has been removed.

Fig. 16 illustrates, in a top down view, a structure having a resist disposed on a silicon oxide layer such that portions of the silicon oxide layer remain exposed and define a lower groove width (wi).

Fig. 17 illustrates, in a cross sectional view, the structure of Fig. 16 along arrow A, including an optical element pattern formed in the silicon oxide layer adjacent the exposed silicon oxide layer portions.

Fig. 18 illustrates the structure of Fig. 16 wherein the exposed silicon oxide layer has been removed to expose portions of the silicon substrate.

Fig. 19 illustrates, in a cross sectional view, the structure of Fig. 18 along arrow A.

Fig. 20 illustrates the structure of Fig. 18 wherein the resist has been removed and a second resist is disposed on the silicon oxide layer such that portions of the silicon oxide layer adjacent the exposed silicon substrate portions remain exposed and define an upper groove width (w_u).

Fig. 21 illustrates, in a cross sectional view, the structure of Fig. 20 along arrow A.

Fig. 22 illustrates the structure of Fig. 20, where, in sequential order, the exposed silicon substrate portions have been etched, the exposed silicon oxide layer has been removed, and the exposed silicon substrate portions have been etched.

Fig. 23 illustrates, in a cross sectional view, the structure of Fig. 22 along arrow A.

Fig. 24 illustrates the structure of Fig. 22 wherein the second resist has been removed. Fig. 25 illustrates, in a cross sectional view, the structure of Fig. 24 along arrow A.

Fig. 26 illustrates the structure of Fig. 24 wherein the optical element pattern has been etched into the substrate.

Fig. 27 illustrates, in a cross sectional view, the structure of Fig. 26 along arrow A.

Fig. 28 illustrates the structure of Fig. 26 wherein the silicon oxide layer has been removed.

Fig. 29 illustrates, in a cross sectional view, the structure of Fig. 28 along arrow A.

Detailed Description of the Preferred Embodiments

The invention provides a structure that allows improved optical fiber positioning and optical coupling/alignment. An embodiment of the structure of the invention is illustrated in Fig. 5. As shown in this figure, substrate 10 has a two- level U-shaped groove extending from the substrate surface 12. More specifically, the groove has a bottom 16, lower side walls 18 having a depth (di) and defining a lower groove width (wi), and upper side walls 17 having a depth (du) and defining an upper groove width (Wu). The bottom 16 is substantially parallel to substrate surface 12. The upper side walls 17 and lower side walls 18 are substantially parallel to substrate surface 12.

Substrate 10 is formed such that an optical fiber, or another type of waveguide, positioned in the two-level U-shaped groove is self-aligned with a second optical fiber (or other type of waveguide). The two-level U-shaped groove of the invention has an upper groove width (w_u) adjacent substrate surface 12, the upper groove width (w») being greater than the width of the optical fiber to be positioned in the groove. In addition, the two-level U-shaped grove has a lower groove width (wi) adjacent the bottom of the groove that is less than the width of the optical fiber, such that the optical fiber is seated and supported in the lower side wall of the groove.

Fig. 6 illustrates an optical fiber 40 that is positioned in a two-level U-shaped groove of the invention. As shown in Fig. 6, optical fiber 40 has a core 42 and a cladding 44. Optical fibers suitable for use with the invention include those optical fibers that are conventionally used. The upper groove width (w_u) and lower groove width (wi) of the groove can be varied to receive optical fibers having various diameters. These optical fiber include, for example, standard optical fibers which have a diameter of about 125 microns, as well as larger optical fibers, such as those having a diameter of about 300 microns to about 350 microns.

With reference to Fig. 6, the optical fiber 40 positioned in the two-level U- shaped groove has a portion of cladding 44 that extends out of the groove. It should be appreciated that the two-level U-shaped groove can be formed such that an optical fiber, positioned in the groove, is completely below the substrate surface. The depth of the upper and lower groove levels, d_u and di respectively, can be adjusted depending on the desired positioning of the optical fiber within the groove. It is preferred that the depth of the lower groove portion (di) is sufficiently great such that optical fiber 40 does not contact bottom 16, as shown in Fig. 6.

Figs. 7 and 8 are scanning electron micrographs (SEM) illustrating two adjacent two-level U-shaped grooves of the invention. Fig. 7 illustrates an angled top down view of two adjacent two-level U-shaped grooves. Fig. 8 illustrates two adjacent two-level U-shaped grooves having an optical element 28 situated between the grooves.

The invention also provides a method of fabricating one or more two-level U- shaped grooves in a substrate. The method includes the step of etching a silicon substrate to form a U-shaped groove extending from one surface of the substrate a depth (di). The groove has a bottom substantially perpendicular to the surface and a lower side wall adjacent the bottom, the lower side wall being substantially perpendicular to the surface and defining a lower groove width (wi). Following this step, the substrate is further etched at the U-shaped groove a depth (du) to form an upper side wall adjacent the surface, the upper side wall being substantially perpendicular to the surface and defining an upper groove width (wu) superimposed over the lower groove width (wi), wherein the upper groove width (w_u) is greater than the lower groove width (wi).

A preferred method of forming the two-level U-shaped groove of the invention is illustrated in Figs. 9-15. The process begins by forming a silicon oxide layer 20 on a surface 12 of substrate 10. The substrate can be any of those conventionally used in the art, and is preferably a silicon wafer. Next, a resist 30 is formed on silicon oxide layer 20 such that a portion 22 of silicon oxide layer 20 remains exposed and defines a lower groove width (wι). The resulting structure is illustrated in Fig. 9. Resist 30 can be selected from resists commonly used in the art, and is preferably a photoresist.

The lower groove width (wi) is dependent upon the diameter of the optical fiber that will be positioned in the two-level U-shaped groove. One of skill in the art can select the width and depth of the two-level U-shaped groove to achieve desired positioning of the optical fiber within substrate 10, such as the positioning illustrated in Fig. 6. When the two-level U-shaped groove is fabricated to contain a standard 125 micron optical fiber, a preferred lower groove width (wi) is about 88 microns.

In the next step of the process of the invention, exposed portion 22 of silicon oxide layer 20 is removed to expose a portion 14 of the silicon substrate. The resulting structure is illustrated in Fig. 10. Preferably, portion 14 is removed by a dry etching technique, such as a plasma etch.

Following removal of exposed portion 22, resist 30 is removed and a second resist 32 is formed on silicon oxide layer 20 such that a portion 24 of silicon oxide layer 20 remains exposed adjacent exposed silicon substrate portion 14 and defines an upper groove width (w_u) superimposed over lower groove width (wi). The resulting structure is illustrated in Fig. 11. The upper groove width (wu) is dependent upon the diameter of the optical fiber that will be positioned in the two-level U-shaped groove. One of skill in the art can select the width and depth of the two-level U-shaped groove to achieve the desired positioning of the optical fiber within substrate 10. When the two-level U- shaped groove is fabricated to contain a standard 125 micron optical fiber, the upper groove width (wu) is preferably about 128 microns.

In the next step of the process of the invention, exposed silicon substrate 14 is etched a depth (di). As indicated in Fig. 12, the etch occurs along arrows B. The etching technique used must be selective to exposed silicon substrate 14 over silicon oxide layer 20 and second resist 32. Silicon oxide layer 20, including exposed portion 24, and second resist 32 serve to protect the underlying portions of substrate 10 from being etched. The second resist 32 and silicon oxide layer 20 must be sufficiently thick such that the dry etch does not etch through to the protected substrate 10. The minimal thickness of the second resist 32 and silicon oxide layer 20 is dependent on the type of etch.

Preferably, exposed silicon substrate 14 is etched using a Bosch etch, which is an etching technique known in the art and described in U.S. Patent No. 5,501,893 issued to Laermer et al. (assigned to Robert Bosch GmbH), herein incorporated by reference. The Bosch etch is a preferred etching technique because it exhibits a high selectivity for silicon over silicon oxide (about 200: 1) and photoresist (about 70: 1), and permits control of the depth (di) of the etch.

Next, exposed portion 24 of silicon oxide layer 20 is removed. The resulting structure is illustrated in Fig. 13. Exposed portion 24 can be removed using conventional techniques that are known in the art. Preferably, portion 24 is removed by a dry etching technique, such as BHF.

Following removal of exposed portion 24, exposed silicon substrate 14 is etched a depth (du). Preferably, exposed silicon substrate 14 is etched using the Bosch etch technique, described above. As indicated in Fig. 14, the etch occurs along arrows B. The etching technique used must be selective to exposed silicon substrate 14 over silicon oxide layer 20. Silicon oxide layer 20 serves to protect the underlying portions of substrate 10 from being etched. Second resist 32 can optionally be removed prior to etching exposed silicon substrate 14, as shown in Fig. 14, using conventional techniques such as dry etching or use of a solvent. The resulting structure, following etching and removal of second resist 32 and oxide layer 20, is illustrated in Fig. 15.

The process of the invention may also be combined with optical element patterns obtained, for example, by optical lithography, electron beam (e-beam) lithography, and x-ray lithography, which are used to fabricate optical elements optically coupled to the optical fibers. The self-aligned structure of the two-level U- shaped groove allows a variety of optical elements to be placed between the optical fibers positioned within the grooves. This may be done in an optical bench format, where patterns etched into the substrate are used for placement of micro-optical elements/components such as lenses, wavelength filters, beamsplitters, polarization optics, etc. Alternatively, the optical elements themselves can be fabricated in the substrate and could include, for example, filters based on 2-D or 1-D periodic structures. For example, a filter might consist of a 1-D Bragg grating consisting of alternating layers of silicon and air (or another material).

Figs. 16-29 illustrate a preferred process for fabricating two adjacent two- level U-shaped grooves in a substrate, the substrate also having an optical element formed in the substrate adjacent the grooves.

Fig. 16 illustrates a resist 30 and exposed silicon oxide layers 22 that define the lower groove width (wi) of the two grooves. The structure illustrated in Fig. 16 can be fabricated using the steps described above with respect to Fig. 9. As described above, the process begins by forming a silicon oxide layer on a surface of a substrate. Next, a resist is formed on the silicon oxide layer such that a portion of the silicon oxide layer remains exposed and defines a lower groove width (wi) . In the case of fabricating two grooves, the resist is formed on the silicon oxide layer such that two portions of the silicon oxide layer remain exposed, each portion defining a lower groove width (wi).

Fig. 17 illustrates a cross section of Fig. 16 along arrow A. As shown in Fig. 17, an optical element pattern 26 has also been formed in silicon oxide layer 20. The optical element pattern 26 can be formed using conventional techniques, such as electron beam (e-beam) lithography or x-ray lithography. The optical element pattern 26 will be used to form an optical element in the substrate 10, as described below. The resist 30 is formed such that exposed portions 22 of silicon oxide layer 20 are aligned with optical element pattern 26, such that optical fibers contained in the two- level U-shaped grooves formed during subsequent processing will be aligned with the optical element.

One of the problems encountered by the fabrication of small feature optical elements, such as photonic crystals, is the incompatibility of KOH processing in V- shaped groove fabrication with fabrication of optical element features. The process of the invention can be combined with optical element patterns which are protected during fabrication of the two-level U-shaped groove. The optical element pattern 26 illustrated in Fig. 17 is protected during processing of the two-level U-shaped grooves by resists 30 and 32 (shown, additionally, in Figs. 19, 21, and 23).

Fig. 18 illustrates the structure of Fig. 16 wherein exposed silicon oxide layer portions 22 have been removed to expose substrate portions 14. During removal of exposed silicon oxide layer portions 22, optical element pattern 26 is protected by resist 30, as shown in Fig. 19, which is a cross sectional view of Fig. 18 along arrow A.

Fig. 20 illustrates the structure of Fig. 18 wherein resist 30 has been removed and a second resist 32 has been formed on silicon oxide layer 20, such that portions 24 of silicon oxide layer 20 adjacent exposed silicon substrate 14 are exposed and define upper groove widths (wu). As shown in Fig. 21, which is an cross sectional view of Fig. 20 along arrow A, optical element pattern 26 is protected by second resist 32.

Fig. 22 illustrates the structure of Fig. 20, where, in sequential order, the steps illustrated with respect to Figs. 12-14 have been performed, namely, etching of exposed silicon substrate portions 14 a depth (di), removal of exposed silicon oxide layer portions 24, and further etching of exposed silicon substrate portions 14 a depth (du) to form two two-level U-shaped grooves. As shown in Fig. 23, which is an cross sectional view of Fig. 22 along arrow A, optical element pattern 26 remains protected by second resist 32.

Next, second resist 32 is removed using conventional techniques. The resulting structure is illustrated in Fig. 24. Fig. 25 illustrates a cross sectional view of the structure of claim 24 along arrow A.

Following removal of second resist 32, optical element pattern 26, which is now exposed, is etched to define an optical element 28 in substrate 10. The optical element pattern can be etched, for example, using a chlorine plasma. Fig. 27 illustrates the structure of Fig. 26 wherein optical element pattern 26 has been used to form optical element 28 in substrate 10.

In the next step of the process of the invention, silicon oxide layer 20 is removed. Silicon oxide layer 20 can be removed using conventional techniques known to those of skill in the art. The resulting structure is illustrated in Fig. 28. Fig. 29 illustrates a cross sectional view of Fig. 28 along arrow A. As shown in Figs. 28 and 29, optical element 28 is positioned between and aligned with the two- level U-shaped grooves.

The invention provides a process that does not suffer from the undercutting problems associated with KOH etching. The process of the invention is also independent of silicon wafer orientation. Furthermore, the process is compatible with other processes used in the art. Through the design of the two-level U-shaped grooves, the optical fiber can be placed securely to within the lateral resolution (about

50 nm) of the etch, despite slight variations in fiber diameter.

With the use of silicon oxide and resist as masks on the substrate, this structure is compatible with most micro-processing techniques. This processing of U- grooves may also be used for most two-level patterns with a deep feature size.

The two-level U-shaped groove provides precise position control of the optical fiber with respect to another optical fiber or an optical device. The control over the etch depth of the upper groove (du) gives the exact position of the fiber relative to the substrate surface.

It will therefore be understood that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.

Claims

ClaimsWhat is claimed is:

1. A structure for optically coupling optical devices, said structure comprising a substrate having a groove formed in one surface thereof, said groove comprising: a bottom substantially parallel to the surface; an upper side wall adjacent the surface, said upper side wall being substantially perpendicular to the surface and defining an upper groove width (w_u); and a lower side wall adjacent the bottom, said lower side wall being substantially perpendicular to the surface and defining a lower groove width (wi), wherein said upper groove width (w_u) is greater than said lower groove width

2. The structure according to claim 1 wherein said substrate comprises silicon.

3. The structure according to claim 1 wherein said groove is a two-level U- shaped groove.

4. The structure according to claim 1 wherein said groove receives an optical device and said lower side wall seats and supports said optical device in a desired position along said substrate.

5. The structure according to claim 1 wherein said upper groove width (Wu) is from about 128 microns.

6. The structure according to claim 1 wherein said lower groove width (wi) is about 88 microns.

7. The structure according to claim 1 wherein a first optical device and a second optical device are optically coupled in said groove.

8. The structure according to claim 7 wherein at least one of said first and said second optical devices is an optical fiber.

9. The structure according to claim 7 wherein said first and second optical devices are optical fibers.

10. The structure according to claim 7 wherein an optical element is positioned between said first optical device and said second optical device.

11. A method for fabricating a two-level U-shaped groove structure for optically coupling optical devices, said method comprising the steps of:

(a) forming a silicon oxide layer on a surface of a silicon substrate; (b) forming a resist on said silicon oxide layer such that a portion of said silicon oxide layer is exposed to define a lower groove width (wi);

(c) removing the exposed portion of said silicon oxide layer to expose a portion of said silicon substrate;

(d) removing said resist; (e) forming a second resist on said silicon oxide layer such that a portion of said silicon oxide layer adjacent said exposed silicon substrate is exposed to define an upper groove width (wu) superimposed over said lower groove width (wi);

(f) etching said exposed silicon substrate;

(g) removing the exposed portion of said silicon oxide layer; and (h) etching said exposed silicon substrate.

12. The method according to claim 11 wherein the lower groove width (wi) is about 88 microns.

13. The method according to claim 11 wherein said removing of steps (c) and (g) is dry etching.

14. The method according to claim 11 wherein the upper groove width (wu) is about 128 microns.

15. The method according to claim 11 wherein said etching in step (f) is Bosch etching.

16. The method according to claim 11 wherein said etching in step (h) is Bosch etching.

17. The method according to claim 11 further comprising the step of removing said second resist between steps (g) and (h).

18. A method for fabricating a structure for optically coupling optical devices, said method comprising the steps of:

(a) etching a silicon substrate to form a U-shaped groove extending from one surface of said substrate a depth (di), said groove having a bottom substantially perpendicular to said surface and a lower side wall adjacent the bottom, said lower side wall being substantially perpendicular to the surface and defining a lower groove width (wi); and

(b) further etching the substrate at said U-shaped groove a depth (du) to form an upper side wall adjacent the surface, said upper side wall being substantially perpendicular to the surface and defining an upper groove width (Wu) superimposed over said lower groove width (wi),

wherein said upper groove width (w_u) is greater than said lower groove width

19. The method of claim 18 wherein said etching in step (a) is Bosch etching.

20. The method of claim 18 wherein said etching in step (b) is Bosch etching.